The present invention relates to a plastic material screw used for an injection molding machine, an extruder or a blow molding machine or the like, and more particularly, to a barrier screw having a subflight which is a type of barrier for separating melted material from unmelted material.
Recently, a barrier screw which comprises a standard single threaded full flight and a subflight, is used for improving the plasticate capacity and the mixing effect of the material. This type of barrier screw has the subflight in a melt zone of a middle portion of the screw. According to the conventional barrier screw, plastic material gradually melted in the melt zone is separated from unmelted solid material, and then the solid material starts to melt. After all of the material has been melted, the melted material is fed to a metering zone located at a most downstream portion of the screw.
In a conventional barrier screw having a subflight, the melted material passes over the top of the subflight and then the melted material is mixed together in a melt channel defined by the subflight. However, the conventional screw has the following drawbacks.
Namely, in the conventional barrier screw, a barrel internal pressure in a solid channel defined by the full flight is high and on the other hand, a barrel internal pressure in the melt channel and in particular a barrel internal pressure at the start portion of the melt channel are low. Furthermore, as the subflight functions as a gas barrier, gas from the melted material in the melt channel cannot be sufficiently purged. Therefore, defects such as silver streaks or the like are formed on the surfaces of moldings.
In the case of a plastic material capable of being easily resolved, burning spots may be caused in the melt channel because of material remaining in the melt channel. Furthermore, as the material moves from the solid channel into the melt channel immediately after being melted, the material and the pigment contained therein cannot sufficiently mix in the solid channel. Therefore, the color dispersion of the pigment is not sufficiently performed.
In order to eliminate the above mentioned drawbacks, there has been proposed a barrier screw which has another mixing barrier in the metering zone in addition to the subflight. However, this type of barrier screw is too long and too expensive because it is required to form double-threaded long grooves on the screw by using sophisticated working technology and many man-hours.